Category 5 is a type of wire known as twisted pair so called because it contains a set of wires inside (in the case of Cat 5, 8 wires or 4 pairs) seperated into pairs each of which is twisted about itself. (Incidentally, cat3 is the same idea except is only "spec"ed for 10mbps instead of 100. Telephone cabling uses the same idea as well, except telephones only require 1 pair.).

Each pair is associated with a color. One of orange, blue, green and brown. Each pair has one wire which is mostly the pair color with specks of white and one wire which is mostly white with specks of the pair color. This is, obviously, used to check which wire you're putting in which pin when crimping cable.

The category 5 standard states that twists in the pairs must occur at least every 1/2 inches but in normal cables the twist occurs much more often. One half inch is generally how far back you can untwist the pairs in order to crimp them at the ends.

Even though it doesn't matter what order you crimp down the pair wires, as long as its the same on both ends of the cable, there do exist two standard pin arrangements known as 568A and 568B. They're both described on the Crimping Cat-5 node. Its best to choose one of them and stick to it throughout one networking job, just in the interest of uniformity. It'll make the job much easier.

Category 5 cabling does have a limit on the degree of turns that it can take. Although its not nearly as drastic as the limit on fiber, anything sharper than the circumfrnce of a silver dollar should be avoided if possible.

Although I'm not sure about this, I seem to remember the reason behind the twisted part being that twisting the wires helped insulate the data travelling on the wire from EM radiation that might otherwise disrupt traffic.

Cat-5 means category 5, and describes a certain kind of twisted pair copper cabling. It is commonly written cat5 or CAT5. Cat5 is the current "standard" for 100 megabit per second network wiring. Depending on implementation it can also serve for both 10Mbps and 1Gbps ("gigabit") ethernet networking connections, 4 or 16Mbps token ring networks, POTS phone cable, ISDN connections, T1 connections, and many others.

The most common use for category 5 cable besides POTS phone networks, or even proprietary phone systems, is for 100Mbps ethernet networks. You can get good transfer rates using 100Mbps ethernet with cat5 over distances as long as 300 meters.

While it doesn't strictly matter which pairs you hook up (as long as you're consistent and you use the wires in pairs) there is a "proper" conventional way to utilize category 5 cable with multiple pairs. The pairs are meant to be used in the following order for POTS service:

white/blue

white/orange

white/green

white/brown

white/slate

The white wire always comes first. You can remember this as "BLOG" BLue, Orange, Green, and brown gets left over. When attaching these to a jack or punchdown block they are always punched down in order. Old cables with green, red, yellow, and black wires should be punched down green/red, and black/yellow, in that order, and would be matched to new wires (if a splice was necessary) in that order.

If your cable has less than five pairs (four pairs is most common for network cabling, as this is what 10base- and 100base-T ethernet networks utilize) then they will appear in their numbered order (I.E. three pair cables have neither brown nor slate wires.) The "white" wire is commonly white with a colored stripe to aid in identifying pairs, though when cutting cable there should never be any exposed wires without the outer jacket. All wires should terminate in junction boxes with a wall plate.

For ethernet, there are two wiring standards: EIA/EIA 568A, and 568B. In both, white wires alternate with colored wires, and the blue pair is always in the center. The white wire comes first, just as for phones. The difference between the two is where the pairs are located. 568A, from pins 1 to 8, runs white/green, green, white/orange, blue, white/blue, orange, white/brown, brown. 568B runs white/orange, orange, white/green, blue, white/blue, green, white/brown, brown. Note that in both types the blue and brown wires do not move, the only difference is the position of green and brown wires. Wiring one end of a cable for 568A and the other for 568B will produce a non-full-duplex crossover cable. For more on ethernet and cat5, see Crimping Cat-5.

Category 5 cable is intended for use with a 100Mhz signal. Using Ethernet this provides a maximum (theoretical) throughput of 100Mbit/s per pair (100Mbps in each direction for full duplex operation.) This cable is intended for use with all types of nbase-T ethernet up to and including 100Base-TX.

The standards for category 5 cable are laid out in ISO/IEC-11801 and TIA/EIA-568-A-5.